Just to clarify, that’s a regulatory definition (CFR) not a statutory one (USC). So, in this case ATF wrote the definition and it still lost. I’m not sure if they did a comment period on that.

Not certain, but I believe in order to be codified in the CFR (Code of Federal Regulations) a regulation must first have gone through the entire public comment period/process.

Regulations in the CFR are regulations, not statutes, but they carry the weight of law. This is unlike various BATFE "interpretations," which appear and disappear based on the tide, the phase of the moon, and the whims of the person occupying the White House.